1. Field of the Invention
The invention relates to an X-ray apparatus, including an apparatus section which is pivotable about a horizontal pivotal axis that does not extend through its centre of gravity, and a counterbalance spring assembly for compensation of the torque which is exerted by the weight of the apparatus section on a coupling member which moves in an orbit about the pivotal axis during pivoting of the apparatus section. Compensated for so that counterbalancing has been achieved, an operator can also pivot a heavy apparatus section with little effort.
2. Description of the Related Art
An X-ray apparatus of this kind is known from EP-A1-0 498 255. In the known X-ray apparatus, the apparatus section, being an X-ray unit of a mammography apparatus, is rigidly connected to a crank-like coupling member which moves in an orbit about the pivotal axis during pivoting of the apparatus section. The coupling member acts, via a roller, on a member which comprises a horizontal running surface for the roller and which can be displaced in the vertical direction. A counterbalancing system acts on this member. When the balancing force delivered by this system is constant, the torque exerted by the apparatus section relative to the pivotal axis can be compensated for, provided that a suitable design has been chosen. A constant balancing force could be delivered by a counterbalancing weight.
However, when the counterbalancing system utilizes a spring assembly, the latter changes its length during pivoting of the apparatus section, and hence the force produced by the counterbalancing system also changes. The torque, however, can then still be fully compensated for in one or two pivotal positions. Moreover, in the known X-ray apparatus the counterbalancing system serves not only to compensate for the torque exerted during pivoting of the apparatus section, but also for the weight of the apparatus section. The force delivered by the counterbalancing system, therefore, should correspond to the weight of the apparatus section or, when the system is connected to the section via a system of pulleys, it should relate to the force according to an integer ratio. In each case the variation of the length of the spring assembly corresponds to the displacement of the apparatus section in the vertical direction. Therefore, it can be comparatively large and cause correspondingly large variations of the spring force produced by the counterbalancing system, so that the torque produced for compensation may deviate substantially from the moment exerted by the apparatus section.